1. Field of the Invention
The present invention relates to a method for forming spacers. More particularly, the present invention relates to a method for forming spacers of different sizes.
2. Description of the Prior Art
In the manufacture process of semiconductors, a pair of protective, self-aligned spacers is usually required to be formed by two sides of the elements in semiconductor devices. Spacers of different sizes are formed for different groups of elements depending on different situations. In order to go with the entire process, the method for forming spacers of different sizes is usually that spacers of uniform size are first formed for different groups of elements after a preliminary etching step, and spacers of different sizes are later formed for a specific group of elements after a secondary etching step.
Please refer to FIGS. 1-5, which illustrate the process for forming spacers of different sizes by two etching procedures. As shown in FIG. 1, there are different groups of elements, such as a first element 110 and a second element 120, formed on a substrate 101 in advance. In addition to the first element 110 and the second element 120, there is a thin oxide layer 130 covering the substrate 101, the first element 110 and the second element 120, a nitride layer 140 covering the thin oxide layer 130, as well as another oxide layer 150 covering the nitride layer 140.
As shown in FIG. 2, a first etching procedure is performed using the nitride layer 140 as an etching stop layer. By adjusting process parameters such as etching time, the first etching may remove part of the oxide layer 150, so that the remaining oxide layer 150 forms a first spacer 151 by the first element 110, and a second side wall 152 by the second element 120. Due to process uniformity and etching selectivity considerations, the total effect of the first etching will not only entirely remove the oxide layer 150 between the first spacer 151 and the second side wall 152, but will also expose part of the nitride layer 140 underneath, turning it into a damaged nitride layer 141 due to the attack of the first dry etching. Compared with the original nitride layer 140, the damaged nitride layer 141 is relatively loose and not dense.
Afterwards, a second selective etching is performed to form spacers of different sizes. Before the second selective etching, as shown in FIG. 3, a patterned photoresist 160 is usually used to cover the region which is not subject to the second etching, such as the specific group of the first element 110. Simultaneously, the patterned photoresist 160 also covers part of the damaged nitride layer 141.
At this point, as shown in FIG. 4, a wet etching is usually used as the second selective etching to remove the second side wall 152. Because the medium of the wet etching is liquid and only part of the damaged nitride layer 141 is covered by the patterned photoresist 160, the etchant of the wet etching will not only remove the second side wall 152, but will also etch the nearby first spacer 151 through the border of the photoresist 160 and the damaged nitride layer 141 by taking advantage of the loose damaged nitride layer 141. In such a way, as shown in the top view of FIG. 5, the region which is not intended to be subject to the second etching and covered by the patterned photoresist 160 is corroded by the etchant too, so that the originally symmetrical first spacer 151 turns into debris 151′ after the photoresist 160 is removed. This is a structural defect beyond remedy for the elements and it seriously hinders following processes.
Because it is the loose damaged nitride layer 141 that causes the wet etchant to have the opportunity to damage the first spacer 151 of the first element 110, and further destroy the method for selectively forming spacers of different sizes, a modified method is therefore needed to prevent the unexpected behavior of etching from the second etching for selectively forming spacers of different sizes.